Out of the box, #sidenav-overlay's z-index is set to 997, and even though the .side-nav class' z-index is set to 999, it competes and ends up covering the .side-nav making it unclickable.
I've been able to set #sidenav-overlay to z-index:950; and that doesn't disrupt the function of the overlay covering the rest of the site while fixing the issue of allowing the .side-nav to be clickable.
I had this same problem on two different sites I built. Otherwise, I love this CSS framework. It's my go to.
Did you already try to move the sidenav ul outside of its container(s)/wrapper(s)?
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Did you already try to move the sidenav ul outside of its container(s)/wrapper(s)?